If it is considered discrimination to not let gays marry then why is it not discrimination to not let this man marry his girlfriend or Mohammed marry a five year old. I think it is a very slipper slope, because the rational behind gay marriage is very flimsy.
Well, the rationale is, quite simply, one of being able to consent.
As long as two adults can consent to enter into a mutually agreeable relationship without coercion or abuse, then that's fine.
The same reason is why child-adult relationships, or animal-human sexual relationships, are not fine or legal, as society considers that the child and the animal do not have the capacity to consent to the relationship having made an informed decision.
For reasons I've given above, I feel that the "consent" argument involving animals is hypocritical because we do a whole lot of exploitative things to animals which don't require their informed consent but suddenly, when it becomes sexual, we say "Oh no, that animal doesn't have the capacity to consent so it's wrong."
However, despite that, I don't buy the "slippery slope" argument because of the issue of consent. There is a clear, bright line between the ability of two consenting adults, whether homosexual or heterosexual, to make an informed, consensual decision, and the ability of children or animals to make an informed, consensual decision.